This invention relates to read and refresh timing of image signals of a color image pickup device (CCD image sensor) which is used as a picking up unit in a device for measuring convergence of a color cathode ray tube (hereinafter referred to as the CRT).
Various devices for measuring convergence have been proposed hitherto to enable convergence adjustment of a color CRT in a quantitative manner. In such devices, a test pattern for measurement is presented on a color CRT, separated into three primary color patterns through optical filters, and picked up by device. Then, the luminous center of each color pattern is calculated to obtain a convergence assessment value.
As an example, Japanese Unexamined Patent Publication 59-74780 discloses a device which first presents an entire white pattern on a CRT to be measured. The white pattern within a specified area is broken into red, blue and green patterns by the respective color separating filters, and picked up by an industrial TV (ITV) camera. The device obtains positional information of individual phosphor dots from these color patterns. Next, a white pattern for convergence assessment is presented on the CRT and picked up without using the color separating filters this time. Based on the positional information of the phosphor dots of each color obtained earlier in the process, the white pattern picked up is separated into red, blue and green patterns, from which luminous centers of gravity of individual color patterns are calculated. Deviation of the positions of the luminous centers of gravity is then obtained as an assessment value of misconvergence.
The example of a conventional device quoted above requires color separating filters as well as an ITV camera. Accordingly, it would be understood that this type of conventional device has a large mechanical size. Furthermore, in such a device of the prior art, the test pattern must be picked up more than once, inevitably requiring a longer measuring time.
To overcome these and other disadvantages of conventional devices, Japanese Patent Application No. 62-259088 has been filed which proposes a device to enable quick assessment of misconvergence. This proposed device employs a CCD image sensor provided with color separating filters as an image pickup device. It picks up the white pattern within a specified sampling area only once, substantially reducing the time required for assessment of misconvergence.
In the meantime, when a CCD image sensor is used as an image pickup device as in the above example, the drive circuit which causes the CRT to be measured to present a test pattern, and the drive circuit which causes the CCD image sensor to store an image signal and the image memory circuit to read the stored image signal have independent horizontal and vertical synchronization signal sources. Consequently, if the CRT drive circuit and the CCD image sensor drive circuit are not perfectly synchronized with each other, the image signal stored in the CCD image sensor would become unstable in some cases. More specifically, since the color CRT glows at exact intervals, glowing images may not be picked up exactly if the CCD image sensor drive timing that controls image signal storage and readout operations is not kept in perfect synchronism with the color CRT glow timing.
Shown in FIG. 15 is a timing chart where the CCD image sensor driving interval is longer than the color CRT glowing interval. Refresh clock signal RF shown in this timing chart is a reference clock signal which determines the timing of transferring the picture element signal on the imaging area of the CCD sensor to its image storage area. The optical sensor output is an output of an optical sensor which monitors the glowing condition of an area of the color CRT from where the CCD image sensor picks up the glowing pattern. The waveform shaping circuit shapes the output of the above mentioned optical sensor as shown in FIG. 15. Since the test pattern certainly glows once during each output pulse interval of the vertical synchronization signal Vd of the CRT to be measured, the output pulse interval of the waveform shaping circuit coincides with the output pulse of the vertical synchronization signal Vd. In the NTSC system, the output of the waveform shaping circuit and the vertical synchronization signal Vd have a common frequency of 60 Hz because the entire picture area is scanned at a field frequency of 60 Hz. However, it is not necessarily certain that the refresh clock signal RF is synchronized with the vertical synchronization signal Vd because they are produced by their respective sources.
If the CCD image sensor driving interval is longer than the color CRT glowing interval as shown in FIG. 15, it may so happen that two successive glowing images are detected by the optical sensor during image signal integration period T1 (which corresponds to the pulse interval of the refresh clock signal RF) of the CCD image sensor while only one glowing image is detected during integration period T2. In such a case, the integrated result of glow image luminance during integration period T1 would be twice as higher than that of glow image luminance during integration period T2, making an accurate measure of the image signal unobtainable. Furthermore, if the refresh clock pulse immediately following integration interval T3 should concur with the glow image detect timing of the optical sensor, in other words, a new glow image is generated on the screen during the time of transferring the image signal for an old glow image, the image signal for the new glow image comes into the image signal for the old glow image in the process of being transferred to disturb the image signal for the old glow image, consequently making it impossible to obtain the correct image signal. The above synchronization problem is even more significant especially when the test pattern is picked up by replacing red, blue and green filters one after another to obtain individual three color image signals at different timings.